I have a lot of experience with Opana, both IR and ER. The ER, when taken orally, has a BA of about 10-20% at most. However, as it is HIGHLY potent, it still works very well for pain management. It is a definite step up from OxyContin as far as time-release pain medications go.
It will gel up if you add water, so shooting the ER's is a no-no. They can, however, be insufflated quite effectively, which raises the BA to around 45-50%. It is the most euphoric opiate I've tried (just about everything but never seen heroin). However, withdrawals are also incredibly awful. They don't last too long, but while they're going, you just feel like death incarnate. Keep that in mind if you plan on abusing them.
To insufflate, suck the coating off carefully, then break it up and smash with a card. You want it to be as good of a powder as you can get, and then take a knife/razor to it to chop it some more. It doesn't gel in your nose like MS-Contin or Teva OC's, as it turns into more of a "paste". I actually think the ER are better for snorting than the IR, as they stick in your nose and while they release a lot at once, they also seem to last a lot longer (6 or so hours).
I also have experience with IV Opana IR, but that's another story.